Children are soooo selfish, aren't they just? They're always leaving shit behind for parents to clean up, shit like gross underwear, dead childhood pets, erotica so well-hidden that they forgot all about it when they moved out, and, according to the latest and greatest scientific research, fetal cells that they deposit in their moms' brains. "Here, Mom — you deal with these extra cells because I don't need them anymore and my fat baby arms are too weak and ineffectual to gather them up myself." (That's how infants talk when they're disrespecting their mothers.)

Really, the (discernable) problem is with sons. New research that was unfortunately not conducted in Ms. Valerie Frizzle's class (this really seems right up their alley) indicates that cells from fetuses can migrate into their mothers' brains. Mothers and fetuses often exchange cells that can survive in bodies for years, a phenomenon, LiveScience patiently explains to us laypeople, called microchimerism. To discover evidence of fetal cells worming their way into a mother's brain, scientists analyzed the brains of 59 women who'd died between the ages of 32 and 101 for traces of male DNA, which they figured would have come from the cells of sons (female DNA would have been trickier to distinguish from a mother's genes).

Researchers found that nearly two thirds of the women — 37 out of the 59 — had traces of the male Y chromosome in several regions of their brains (the oldest women in which these cells were detected was 94, giving new shape to the maternal wish/fear that a son will never, ever move out). Though a defense system known somewhat crudely as the blood-brain barrier (it just sounds gross) keeps many germs and drugs from throwing wine-drenched Caligula orgies in the brain, doctors have found that this barrier becomes way more permeable during pregnancy, meaning that fetal cells can slip through and nestle forever in the warm bosom of a mother's brain.

The presence of these cells may be good for a mother's health...or it may be terrible. No one is really sure yet. Even though 26 of the women had no signs of brain disorders when they were alive, an eye-catching 33 of them had Alzheimer's disease. Researchers found that women with Alzheimer's were less likely to have male DNA in their brains than women who didn't have such a diagnosis, but the connection, as of right now, seems a little tenuous.

Earlier research on microchimerism suggested that fetal cells might also protect against breast cancer and tissue repair in women, but that they might also increase the risk for colon cancer and help catalyze certain autoimmune diseases. Past studies have also suggested that Alzheimer's is more prevalent in women with a high number of pregnancies than in childless women, so we really need to rev up that Magic School Bus and dive into some brains to figure all this microchimerism nonsense out. Inquiring grade-school minds want to know.